In the field of energy storage using batteries, the properties of the electrodes, and especially of the current collectors that the electrodes include, are an important element as regards the overall performance of the batteries. In order for a material to be able to be used as a collector, it is desirable for it to have a high electronic conductivity, good electrochemical stability and a large area of contact with the active material. Nanomaterials have a high area/volume ratio thereby increasing the reaction rates, by reducing the diffusional limitations, and the use of nanomaterials for production of current collectors is under development.
It is known to prepare nanostructured electrically conductive materials in the form of solid or hollow fibers using a porous membrane either electrochemically or chemically.
A chemical method is described for example by B. Bercu, et al. [Nuclear Instruments and Methods in Physics Research B 225, 497-502, (2004)]. The method consists in activating a polycarbonate membrane and then bringing it into contact with a solution of a copper precursor. After a sufficient contact time, so that copper is deposited on the walls of the pores of the membrane and also forms a layer on the surface of the membrane, the membrane is removed by dissolving it and what is obtained is a self-supporting element consisting of a copper layer bearing nanoelements in the form of hollow copper nanotubes on its surface. However, in that method, the copper layer forming a substrate obtained by electrochemical and/or chemical deposition in the case of the nanoelements is necessarily porous, and the length of the nanoelements in the final self-supporting element is dictated by the thickness of the membrane since the formation of the copper film on the surface of the membrane starts only when the surface of the pores of the membrane is completely covered with copper. Furthermore, the fact that the copper nanoelements on the copper substrate are hollow is unfavorable to the mechanical strength of the self-supporting element, and also to its use as a current collector, the quantity of current conveyed through the hollow elements being less than with solid nanoelements.
Methods are also known for depositing, electrochemically, a metallic coating of nanostructured elements on a conductive substrate. In these methods, it is necessary to pretreat the membrane that will be used as support for the formation of the nanostructured elements, so as to render said membrane conductive. The treatment generally consists in applying a film of noble metal by PVD or CVD to the membrane. This technique is complicated to implement on an industrial scale and makes the entire process expensive. Furthermore, it does not allow a self-supporting element consisting of just one metal to be obtained when the intended metal cannot be applied in film form by PVD or CVD.
For example, D. Dobrev, et al., [Nuclear Instruments and Methods in Physics Research B 149, 207-212, (1999)] describes a method for forming nanoscale metal needles electrochemically using a porous membrane. The method consists in applying a conductive film of Au on one face of a polycarbonate membrane by PVD, in electrodepositing a copper layer on the Au conductive film, then in depositing copper in the pores of the membrane via that face of the membrane that has remained free, and finally in dissolving the membrane using a suitable solvent. The self-supporting element obtained consists of a copper substrate bearing nanoscale copper needles, an Au film being interposed between the copper substrate and the nanoscale needles.
Y. Konishi, et al., [Journal of Electroanalytical Chemistry 559, 149-153, (2003)] describe a method for electrodepositing copper nanowires in a nanoporous polycarbonate membrane. In this method too, a conducting film (Pt—Pd) is deposited beforehand on the membrane, and then copper is electrodeposited. When the pores of the membrane are filled with copper, a copper layer forms on top, which layer acts as a cathode substrate during the electrodeposition. After the membrane has been dissolved, what is obtained is a self-supporting element consisting of a copper substrate bearing copper nanowires on the free end of which there may be a Pt—Pd film. The material of the substrate of such an element obtained by electrodeposition is porous, the porosity being inherent in the deposition process.